Crane supports

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Tower cranes are usually supplied on a hire basis, with the client being responsible for the design and construction of the base upon which the crane will be erected. Details of loading are provided by the crane supplier and the base is most commonly designed as a temporary structure, although sometimes a crane base will be incorporated into the permanent structure to save on cost and time.

Loads are given in two forms, ‘in service’ loads, where the crane is functioning and wind speeds are restricted (ie cranes will not operate at high wind speeds), and ‘out of service’ loads, where the crane is not being used but maximum wind speeds may occur.

The location for a crane should be carefully selected to provide a maximum working radius, and when two cranes are being used on the same site mast heights and jib lengths must be considered so that they do not clash.

Cranes are typically structured around two rails at their base between 4.5m-10m apart with wheels in each corner. Cranes are not normally tied down, so sufficient kentledge must be provided to ensure vertical loading from the crane passes through the rails and into the foundation. The foundation is designed so that the unfactored loading from the crane and the unfactored loading from the foundation itself create a bearing pressure which is less than the allowable bearing pressure of the soil.

Various foundation types can be selected depending on the ground conditions:

• Where possible a structural fill can be compacted and used to support a crane with the load spreading through layers of track support at 45° in to the soil strata below.
• When loads from the crane increase, reinforced concrete foundations may be required. This can involve a series of reinforced concrete beams used to support line loads as a result of the crane loading.
• When ground conditions are particularly poor, piled foundations may be necessary. Careful design is required to ensure that reinforcement at the top of the pile top does not cause problems for positioning the mast base section of the crane.

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[edit] External references

• BS5975:2008 + A1: 2001 Code of Practice for Temporary Works Procedures and the Permissible Stress Design of Falsework (BSI 2011).